Two of the most serious problems in the use of steel strand in prestressing concrete and in stay cables for cable-stayed bridges are corrosion and fatigue, and both have received substantial attention in the art. The corrosion problem needs no explanation, and varies greatly with the environment of the location. Fatigue is the tendency of a material to fail after a large number of repeated loadings at a stress level that would not cause failure for static loading. The mechanism of fatigue is the gradual growth of a crack under the influence of fluctuating stresses until a critical depth is reached and brittle fracture occurs. Any condition leading to a stress concentration can act as a crack initiator, such as material flaws, welds, or surface damage. Bending fatigue, which may be considered as a form of work hardening, can occur at the anchorages of the stay cables of cable-stayed bridges where repetitive bending of the stay cable strand may ultimately cause bending fatigue failure. Fretting fatigue arises from a rubbing action, whether rubbing of strands against each other, or strand against the surrounding duct in posttensioned prestressed concrete, or rubbing of the wires of a strand against each other. For instance, when individual strands or wires slip relative to each other or relative to the duct, this slip, which may be minute, can cause severe abrasions, and increases stresses between the contacting elements which can initiate surface cracks that will propagate under cyclic loading. The problem of metal on metal rubbing under the influence of lateral and fluctuating axial stresses is known as "fretting," and studies have shown that extremely small slip amplitudes such as those that occur between individual wires of a strand, or parallel bundled wires can have a significant impact on the fatigue behavior. This is set out in a published research report by G. P. Wollmann et al entitled "Fretting Fatigue in Post-tensioned Concrete" dated November, 1988, report No. FHWA/TX-90+465-2F, involving research performed by the Center for Transporation Research, Bureau of Engineering Research, The University of Texas at Austin, Austin, Tex., in cooperation with the U.S. Department of Transportation, Federal Highway Administration, and obtainable from National Technical Information Service, Springfield, Va.
A significant advance in the art of corrosion protection, particularly for concrete prestressing strand, occurred with the introduction in late 1982 of epoxy coated strand, which is now well-known and which has been commercially available since about late 1982 or early 1983, and is described in commonly assigned European patent No. 0110542 of Oct. 21, 1983, and in an article entitled "Epoxy Coated Seven-Wire Strand for Prestressed Concrete" appearing in the Journal of the Prestressed Concrete Institute, Volume 29, No. 4, July-August, 1984. The Wollmann et al report mentioned above confirmed also that the epoxy coating improved the fatigue performance of the strand as used in posttensioning strands as involved in the study. Since its introduction, the epoxy coated strand has been used fairly widely in both pretensioned and posttensioned prestressed concrete, and has been used in the stay cables of a major cable-stayed bridge across the Mississippi River at Quincy, Ill., USA, as described in an article captioned "Cable-Stay Bridge to Span Mississippi River" appearing in the November, 1986, issue of Roads & Bridges Magazine. It will be understood, of course, that, unlike a suspension bridge, which supports the deck from vertical cables, a cable-stayed bridge is borne on cables hung at angles from tall towers, for example, as schematically illustrated in FIG. 1 of Jungwirth et al U.S. Pat. No. 4,633,540 and as shown in the above mentioned article in Roads & Bridges Magazine.
The present invention involves a modification of the known epoxy coated strand which will be beneficial under certain circumstances, the modification involving filling of the internal voids or interstices with the epoxy based resin such that any corrosive media which might penetrate the epoxy coating will be prohibited from migrating through the voids or interstices between and along the wires. At the same time, this achieves the additional benefit of resisting relative movement of the wires of the strand, thus increasing the flexural stiffness of the coated and impregnated strand so that it acts in the manner of an integral composite material, and increases the resistance to fretting fatigue. As a still further benefit, the impregnated and coated strand can be made by a new technique involving relatively minor modification of known production lines for making the known epoxy coated strand. We also consider the new technique to be superior to previously proposed techniques for impregnating wire rope, strand or the like.
Prior proposals for impregnating rope or strand have been set out in Wheeler U.S. Pat. No. 4,635,432 issued Jan. 13, 1987, and Campbell U.S. Pat. No. 3,425,207 issued Apr. 24, 1967. The Wheeler patent describes and enumerates the disadvantages of previous methods for impregnating wire rope with plastic material, most of which appear to have involved extrusion techniques. Wheeler proposed a method of continuously impregnating and encapsulating wire rope with a polymer formed from at least two reactive components which when mixed and heated together react chemically to form a low viscosity liquid intermediate polymer which continues to cure until it becomes a solid. In accordance with Wheeler's method, each component is separately pumped into a mixing unit, and the resulting blended mixture is pumped into an injection die. The mixture does not rapidly react to form the solid polymer since heat sufficient to initiate rapid reaction between the components has not been applied to the mixture. The wire rope is preheated to a temperature sufficient to initiate rapid reaction of the components, and the preheated rope is then introduced into the injection die. As the preheated wire rope passes through the injection die in contact with the mixed components, the components are heated and react upon contact with the wire rope to form the liquid intermediate polymer. The liquid intermediate polymer continuously impregnates the interstitial voids of the wire rope and encapsulates the wire rope as it passes through the die. While the Wheeler method may be workable, it appears unduly complex relative to the technique involved in the present invention. Campbell U.S. Pat. No. 3,425,207 involves a method of forming a wire rope in which adjacent wires are separated by an elastomeric material which fills the interstices between the wires, the method involving drawing through a die a strand formed from a mixture of filamentary components of metal and elastomer. The die is a sizing or compacting die, and the components are subjected to tension, radial pressure and temperature corresponding to the softening point of the elastomer whereby the interstitial spaces between the filamentary components are filled by plastic flow of the elastomer. The patent states that the adjacent wires, in the same layer at least, are not in physical contact with each other but are separated by the elastomeric material, and that there is no adhesion of the steel wires to the elastomer such that they are able to move without hindrance, a feature which is contrary to a feature of the present invention, that is, limiting relative movement of the wires so as to enhance resistance to fretting fatigue.